Automatic start-stop vehicles stop the engine to save fuel when the vehicle is stopped or approaching a stop and then automatically restart the engine in anticipation of the vehicle moving. When the engine is stopped, various vehicle components or accessories may be powered by a battery, or the engine may be started in response to a component or accessory load that exceeds the available battery power, which reduces the fuel economy. A low voltage single battery (such as a 12V lead-acid battery) may be used to power the components and accessories, but has limited charge power capability due to the dynamic charge acceptance and battery chemistry and construction constraints. In addition, modern vehicles are equipped with more features and options that consume significant electric power. A single battery topology also makes it difficult to capture and store energy from sources that generate power, to maximize the use of the generated energy and stored energy, and to be able to efficiently use generated and stored energy to power the vehicle technology on demand. Due to the limited power availability, some systems inhibit operation and/or provide limited functionality of various features, components, or accessories when the engine is stopped. Furthermore, during engine cranking battery voltage may drop significantly and affect operation or functioning of various vehicle technology.
Start-stop vehicles may use a lithium-ion (Li-Ion) battery to overcome some of the power issues associated with a low voltage lead-acid battery. The Li-Ion battery may feed the entire power subsystem in the vehicle to provide a stable source of energy and to isolate all subsystems from the effects of engine cranking during auto-stopping, while the vehicle is auto-stopped, and during auto-starting. While providing various advantages, the energy cost (amp-hours) for the Li-Ion battery is relatively high compared to energy cost from a lead-acid battery.
Hybrid vehicles include an engine and an electric machine that operates as a motor/generator with an associated traction battery, which is typically a Li-Ion battery, to provide an electric vehicle (EV) mode using only electric power to propel the vehicle, or a hybrid electric vehicle (HEV) mode that uses the engine and motor to propel the vehicle. The Li-Ion battery for a hybrid vehicle typically has a much larger capacity than the battery for a stop-start vehicle due to its intended use. Hybrid vehicles may start the engine using a dedicated low voltage starter motor and/or various types and sizes of electric machines that may function as a motor/generator or an integrated starter-generator (ISG) with power provided by an associated low voltage battery lead-acid battery, or by the Li-Ion traction battery using associated power electronics and voltage converter. Similar to a stop-start vehicle, the engine may be started frequently under various operating conditions to meet driver demanded torque, to transition between EV and HEV operating modes, or to power vehicle systems or technology. Similar to the use of a Li-Ion battery to power technology in stop-start vehicles, the energy cost (amp-hours) for the Li-Ion battery is relatively high compared to energy cost from the lead-acid battery.